The Adafruit MQTT Library has been updated to version 0.12.0, and now supports MQTT v3.1.1 (both MQTT v3 and v3.1.1 are supported by Adafruit IO). The update is backward compatible, so v0.12.0 will work with your existing sketches. You can grab the latest version using the Arduino v1.6.4+ Library Manager, or download/clone the latest version from the GitHub repository.
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